Behavioral toxicology methodology has often been proposed as a potential way to optimally protect the health of individual workers exposed to neurotoxic substances. In sufficiently sensitive behavioral tests were used, workers could be removed from exposure areas upon showing evidence of the first subtle reversible effects of exposure. However, in the case of industrial mercury exposure, previous studies have been of a large group, cross-sectional nature and have provided little or no basis for use of tests in medical monitoring of individual mercury exposed workers. While two previous studies have produced electromyographic, tremor and psychomotor tests with demonstrated sensitivity to mercury effects, results are on a group basis and wide variability among subjects makes it impossible to establish control limits relevant to individual workers. The research proposed here will consist of a longitudinal study and will concentrate attention on those workers who undergo sharp changes in mercury exposure, due to entering or leaving mercury exposure work areas through job changes. Since the design is "within subjects," meaningful control limits can be established in meeting the goal of developing tests for use in routine medical examinations of exposed workers. Periodic medical examinations are currently recommended because hygiene controls involving monitoring of air and urine mercury cannot insure protection of particularly sensitive individuals. The problem with current medical techniques, however, is the fact that intoxication must reach clinical proportions in order to be ditected. Therefore, the development of behavioral testing methodology sensitive to the first reversible signs is considered the most important research objective. The proposed research is a continuation of a recent (1975) study of 130 chlor-alkali workers. These workers have received behavioral testing and they have recorded baseline test results. They, therefore, represent an ideal group for the continuing longitudinal study.